Atrial fibrillation is the most common significant cardiac arrhythmia in horses. It causes exercise intolerance and welfare harm but responds well to cardioversion in most cases.
Atrial fibrillation causes exercise intolerance that directly affects equine welfare and function. Affected horses show reduced performance capacity — they tire earlier and may show signs of distress during exertion that would previously have been comfortable. The loss of atrial contribution to ventricular filling (approximately 20-25% of cardiac output) reduces exercise capacity and causes respiratory distress under load.
Beyond exercise limitation, AF horses may show resting signs of discomfort — irregular heart rhythm, variable pulse quality, and occasional signs of cardiovascular instability. The welfare impact is variable between individuals, with some horses appearing clinically normal at rest while others show signs of cardiovascular compromise even at low activity levels.
Treatment by cardioversion (restoring normal rhythm) dramatically improves welfare in most horses. Quinidine sulfate administered via nasogastric tube every 2 hours until cardioversion occurs is the standard treatment and is effective in most cases. Horses in sinus rhythm after cardioversion typically return to full exercise capacity and pre-AF welfare status. Long-term prognosis depends on underlying cardiac disease — lone AF has excellent long-term prognosis while AF secondary to cardiac pathology carries more guarded outlook.